452

HISTORY OF ENGINEERING.

Book I.

increase where the inverted or the abutting arches on the piers commence, agreeably to theratio shown in the drawings, and in the abutments they are to be continued on the sameline of radius to the full extremity of the abutments, diminishing from the top of the

ELEVATION OF CENTRE ARCH, LONDON BR1DGB.

abutments until they come to the regular depth of the arches, as shown in the drawings;the outside or quoins of the arches are to continue in length to meet the horizontal coursesin the spandrils between the arches.

“ The arch-stones are all to be dressed perfectly smooth and straight on the beds, sides,and faces, without any deficiency whatever. The faces and soffits are to be fine-dressedand rusticated as before described ; the extrados to be formed according to the drawings,and rough hammer-dressed, except where the inverted arches join, and these are to besmooth dressed, so as to bear and fit solidly in every part.

“ The whole of the arches, interior and exterior, are to be composed of the best granite asbefore described ; there are to be four sets of connecting bars, composed of the very bestwrought-iron in each arch, of the forms, dimensions, and position shown in the drawings;the whole of the arch-stones are to be properly bedded and jointed in mortar hereafter tobe described.

“ Solid spandrils over the piers to the under side of the inverted arches; these arc to bemade up with solid masonry in horizontal courses corresponding in thickness, and closelyfitted to the extrados or back part of the arch-stones at their respective places as shown inthe drawings: these are to be composed of the best granite as before described.

“ The top bed of each course is to be dressed off smooth and even before the next course iscommenced ; the whole of the above courses of stone are to be firmly squared and dressedin their beds and joints, so that they may fit solid and close in every part, and they are tobe set in beds of fine mortar hereafter to be described.

“ Inverted Arches _The inverted arches over the piers are to be constructed as shown in

the drawings, and the depth of these inverted or abutting arches is to be 6 feet in themiddle for the two centre piers, and 5 feet for the two side piers ; these inverted arches areto be composed of courses not less than 18 inches thick at the soffit, and increasing inthickness according to the radius, and they are to rest on the solid spandrils beforedescribed, and to be close and accurately fitted to them and the extremities or extrados of thearch-stones; they are to be composed of the very best granite as before described; the wholeof the above described inverted arch-stones are to be finely dressed in every part withoutany deficiencies whatever ; they are to be set in fine mortar hereafter to be described:there is to be a circular opening of 18 inches diameter through the centre of each pier andthrough the solid spandril and inverted arches, and through the pier below the level oflow-wat® mark, as shown in the drawings, where it is to communicate with the river tocarry off the leakage or soakage water that may accumulate from above.

“ The outside spandril walls , between the arches and against the abutments, are to be 5feet thick, and fronted with granite, as before described, in courses of suitable thickness tothose of the arch-stones, against which they are to abut, and fit closely as represented in thedrawings.